Digital non-Foster-inspired electronics for broadband impedance matching

Narrow bandwidths are a general bottleneck for applications relying on passive, linear, subwavelength resonators. In the past decades, several efforts have been devoted to overcoming this challenge, broadening the bandwidth of small resonators by the means of analog non-Foster matching networks for radiators, antennas and metamaterials. However, most non-Foster approaches present challenges in terms of tunability, stability and power limitations. Here, by tuning a subwavelength acoustic transducer with digital non-Foster-inspired electronics, we demonstrate five-fold bandwidth enhancement compared to conventional analog non-Foster matching. Long-distance transmission over airborne acoustic channels, with approximately three orders of magnitude increase in power level, validates the performance of the proposed approach. We also demonstrate convenient reconfigurability of our non-Foster-inspired electronics. This implementation provides a viable solution to enhance the bandwidth of sub-wavelength resonance-based systems, extendable to the electromagnetic domain, and enables the practical implementation of airborne and underwater acoustic radiators.

Metasurface-enhanced photochemical activity in visible light absorbing semiconductors

Heterogeneous photocatalysis is an important research problem relevant to a variety of sustainable energy technologies. However, obtaining high photocatalytic efficiency from visible light absorbing semiconductors is challenging due to a combination of weak absorption, transport losses, and low activity. Aspects of this problem have been addressed by multilayer approaches, which provide a general scheme for engineering surface reactivity and stability independent of electronic considerations. However, an analogous broad framework for optimizing light-matter interactions has not yet been demonstrated. Here, we establish a photonic approach using semiconductor metasurfaces that is highly effective in enhancing the photocatalytic activity of GaAs, a high-performance semiconductor with a near-infrared bandgap. Our engineered pillar arrays with heights of ∼150 nm exhibit Mie resonances near 700 nm that result in near-unity absorption and exhibit a field profile that maximizes charge carrier generation near the solid-liquid interface, enabling short transport distances. Our hybrid metasurface photoanodes facilitate oxygen evolution and exhibit enhanced incident photon-to-current efficiencies that are ∼22× larger than a corresponding thin film for resonant excitation and 3× larger for white light illumination. Key to these improvements is the preferential generation of photogenerated carriers near the semiconductor interface that results from the field enhancement profile of magnetic dipolar-type modes.

Pseudo-spin switches and Aharonov-Bohm effect for topological boundary modes

Topological boundary modes in electronic and classical-wave systems exhibit fascinating properties. In photonics, topological nature of boundary modes can make them robust and endows them with an additional internal structure-pseudo-spins. Here, we introduce heterogeneous boundary modes, which are based on mixing two of the most widely used topological photonics platforms-the pseudo-spin-Hall-like and valley-Hall photonic topological insulators. We predict and confirm experimentally that transformation between the two, realized by altering the lattice geometry, enables a continuum of boundary states carrying both pseudo-spin and valley degrees of freedom (DoFs). When applied adiabatically, this leads to conversion between pseudo-spin and valley polarization. We show that such evolution gives rise to a geometrical phase associated with the synthetic gauge fields, which is confirmed via an Aharonov-Bohm type experiment on a silicon chip. Our results unveil a versatile approach to manipulating properties of topological photonic states and envision topological photonics as a powerful platform for devices based on synthetic DoFs.

Passive frequency comb generation at radiofrequency for ranging applications

Optical frequency combs, featuring evenly spaced spectral lines, have been extensively studied and applied to metrology, signal processing, and sensing. Recently, frequency comb generation has been also extended to MHz frequencies by harnessing nonlinearities in microelectromechanical membranes. However, the generation of frequency combs at radio frequencies (RF) has been less explored, together with their potential application in wireless technologies. In this work, we demonstrate an RF system able to wirelessly and passively generate frequency combs. This circuit, which we name quasi-harmonic tag (qHT), offers a battery-free solution for far-field ranging of unmanned vehicles (UVs) in GPS-denied settings, and it enables a strong immunity to multipath interference, providing better accuracy than other RF approaches to far-field ranging. Here, we discuss the principle of operation, design, implementation, and performance of qHTs used to remotely measure the azimuthal distance of a UV flying in an uncontrolled electromagnetic environment. We show that qHTs can wirelessly generate frequency combs with μWatt-levels of incident power by leveraging the nonlinear interaction between an RF parametric oscillator and a high quality factor piezoelectric microacoustic resonator. Our technique for frequency comb generation opens new avenues for a wide range of RF applications beyond ranging, including timing, computing and sensing.

Efficient excitation and control of integrated photonic circuits with virtual critical coupling

Critical coupling in integrated photonic devices enables the efficient transfer of energy from a waveguide to a resonator, a key operation for many applications. This condition is achieved when the resonator loss rate is equal to the coupling rate to the bus waveguide. Carefully matching these quantities is challenging in practice, due to variations in the resonator properties resulting from fabrication and external conditions. Here, we demonstrate that efficient energy transfer to a non-critically coupled resonator can be achieved by tailoring the excitation signal in time. We rely on excitations oscillating at complex frequencies to load an otherwise overcoupled resonator, demonstrating that a virtual critical coupling condition is achieved if the imaginary part of the complex frequency equals the mismatch between loss and coupling rate. We probe a microring resonator with tailored pulses and observe a minimum intensity transmission T = 0.11 in contrast to a continuous-wave transmission T = 0.58 , corresponding to 8 times enhancement of intracavity intensity. Our technique opens opportunities for enhancing and controlling on-demand light-matter interactions for linear and nonlinear photonic platforms.

Poincaré sphere trajectory encoding metasurfaces based on generalized Malus' law

As a fundamental property of light, polarization serves as an excellent information encoding carrier, playing significant roles in many optical applications, including liquid crystal displays, polarization imaging, optical computation and encryption. However, conventional polarization information encoding schemes based on Malus' law usually consider 1D polarization projections on a linear basis, implying that their encoding flexibility is largely limited. Here, we propose a Poincaré sphere (PS) trajectory encoding approach with metasurfaces that leverages a generalized form of Malus' law governing universal 2D projections between arbitrary elliptical polarization pairs spanning the entire PS. Arbitrary polarization encodings are realized by engineering PS trajectories governed by either arbitrary analytic functions or aligned modulation grids of interest, leading to versatile polarization image transformation functionalities, including histogram stretching, thresholding and image encryption within non-orthogonal PS loci. Our work significantly expands the encoding dimensionality of polarization information, unveiling new opportunities for metasurfaces in polarization optics for both quantum and classical regimes.

Broadband angular spectrum differentiation using dielectric metasurfaces

Signal processing is of critical importance for various science and technology fields. Analog optical processing can provide an effective solution to perform large-scale and real-time data processing, superior to its digital counterparts, which have the disadvantages of low operation speed and large energy consumption. As an important branch of modern optics, Fourier optics exhibits great potential for analog optical image processing, for instance for edge detection. While these operations have been commonly explored to manipulate the spatial content of an image, mathematical operations that act directly over the angular spectrum of an image have not been pursued. Here, we demonstrate manipulation of the angular spectrum of an image, and in particular its differentiation, using dielectric metasurfaces operating across the whole visible spectrum. We experimentally show that this technique can be used to enhance desired portions of the angular spectrum of an image. Our approach can be extended to develop more general angular spectrum analog meta-processors, and may open opportunities for optical analog data processing and biological imaging.

Chiral transmission by an open evolution trajectory in a non-Hermitian system

Exceptional points (EPs), at which two or more eigenvalues and eigenstates of a resonant system coalesce, are associated with non-Hermitian Hamiltonians with gain and/or loss elements. Dynamic encircling of EPs has received significant interest in recent years, as it has been shown to lead to highly nontrivial phenomena, such as chiral transmission in which the final state of the system depends on the encircling handedness. Previously, chiral transmission for a pair of eigenmodes has been realized by establishing a closed dynamical trajectory in parity-time- (PT-) or anti-PT-symmetric systems. Although chiral transmission of symmetry-broken modes, more accessible in practical photonic integrated circuits, has been realized by establishing a closed trajectory encircling EPs in anti-PT-symmetric systems, the demonstrated transmission efficiency is very low due to path-dependent losses. Here, we demonstrate chiral dynamics in a coupled waveguide system that does not require a closed trajectory. Specifically, we explore an open trajectory linking two infinite points having the same asymptotic eigenmodes (not modes in PT- and anti-PT-symmetric systems), demonstrating that this platform enables high-efficiency chiral transmission, with each eigenmode localized in a single waveguide. This concept is experimentally implemented in a coupled silicon waveguide system at telecommunication wavelengths. Our work provides a new evolution strategy for chiral dynamics with superior performance, laying the foundation for the development of practical chiral-transmission devices.

Acoustic Higher-Order Topological Insulators Induced by Orbital-Interactions

The discovery of higher-order topological insulator metamaterials, in analogy with their condensed-matter counterparts, has enabled various breakthroughs in photonics, mechanics, and acoustics. A common way of inducing higher-order topological wave phenomena is through pseudo-spins, which mimic the electron spins as a symmetry-breaking degree of freedom. Here, this work exploits degenerate orbitals in acoustic resonant cavities to demonstrate versatile, orbital-selective, higher-order topological corner states. Type-II corner states are theoretically investigated and experimentally demonstrated based on tailored orbital interactions, without the need for long-range hoppings that has so far served as a key ingredient for Type-II corner states in single-orbital systems. Due to the orthogonal nature of the degenerate p orbitals, this work also introduces a universal strategy to realize orbital-dependent edge modes, featuring high-Q edge states identified in bulk bands. These findings provide an understanding of the interplay between acoustic orbitals and topology, shedding light on orbital-related topological wave physics, as well as its applications for acoustic sensing and trapping.

Floquet parity-time symmetry in integrated photonics

Parity-time (PT) symmetry has been unveiling new photonic regimes in non-Hermitian systems, with opportunities for lasing, sensing and enhanced light-matter interactions. The most exotic responses emerge at the exceptional point (EP) and in the broken PT-symmetry phase, yet in conventional PT-symmetric systems these regimes require large levels of gain and loss, posing remarkable challenges in practical settings. Floquet PT-symmetry, which may be realized by periodically flipping the effective gain/loss distribution in time, can relax these requirements and tailor the EP and PT-symmetry phases through the modulation period. Here, we explore Floquet PT-symmetry in an integrated photonic waveguide platform, in which the role of time is replaced by the propagation direction. We experimentally demonstrate spontaneous PT-symmetry breaking at small gain/loss levels and efficient control of amplification and suppression through the excitation ports. Our work introduces the advantages of Floquet PT-symmetry in a practical integrated photonic setting, enabling a powerful platform to observe PT-symmetric phenomena and leverage their extreme features, with applications in nanophotonics, coherent control of nanoscale light amplification and routing.

Spatio-temporal coupled mode theory for nonlocal metasurfaces

Diffractive nonlocal metasurfaces have recently opened a broad range of exciting developments in nanophotonics research and applications, leveraging spatially extended-yet locally patterned-resonant modes to control light with new degrees of freedom. While conventional grating responses are elegantly captured by temporal coupled mode theory, current approaches are not well equipped to capture the arbitrary spatial response observed in the nascent field of nonlocal metasurfaces. Here, we introduce spatio-temporal coupled mode theory (STCMT), capable of elegantly capturing the key features of the resonant response of wavefront-shaping nonlocal metasurfaces. This framework can quantitatively guide nonlocal metasurface design while maintaining compatibility with local metasurface frameworks, making it a powerful tool to rationally design and optimize a broad class of ultrathin optical components. We validate this STCMT framework against full-wave simulations of various nonlocal metasurfaces, demonstrating that this tool offers a powerful semi-analytical framework to understand and model the physics and functionality of these devices, without the need for computationally intense full-wave simulations. We also discuss how this model may shed physical insights into nonlocal phenomena in photonics and the functionality of the resulting devices. As a relevant example, we showcase STCMT's flexibility by applying it to study and rapidly prototype nonlocal metasurfaces that spatially shape thermal emission.

Gate-Tuning Hybrid Polaritons in Twisted α-MoO3/Graphene Heterostructures

Modulating anisotropic phonon polaritons (PhPs) can open new avenues in infrared nanophotonics. Promising PhP dispersion engineering through polariton hybridization has been demonstrated by coupling gated graphene to single-layer α-MoO3. However, the mechanism underlying the gate-dependent modulation of hybridization has remained elusive. Here, using IR nanospectroscopic imaging, we demonstrate active modulation of the optical response function, quantified in measurements of gate dependence of wavelength, amplitude, and dissipation rate of the hybrid plasmon-phonon polaritons (HPPPs) in both single-layer and twisted bilayer α-MoO3/graphene heterostructures. Intriguingly, while graphene doping leads to a monotonic increase in HPPP wavelength, amplitude and dissipation rate show transition from an initially anticorrelated decrease to a correlated increase. We attribute this behavior to the intricate interplay of gate-dependent components of the HPPP complex momentum. Our results provide the foundation for active polariton control of integrated α-MoO3 nanophotonics devices.

Observation of Room-Temperature Exciton-Polariton Emission from Wide-Ranging 2D Semiconductors Coupled with a Broadband Mie Resonator

Two-dimensional exciton-polaritons in monolayer transition metal dichalcogenides (TMDs) exhibit practical advantages in valley coherence, optical nonlinearities, and even bosonic condensation owing to their light-emission capability. To achieve robust exciton-polariton emission, strong photon-exciton couplings are required at the TMD monolayer, which is challenging due to its atomic thickness. High-quality (Q) factor optical cavities with narrowband resonances are an effective approach but typically limited to a specific excitonic state of a certain TMD material. Herein, we achieve on-demand exciton-polariton emission from a wide range of TMDs at room temperature by hybridizing excitons with broadband Mie resonances spanning the whole visible spectrum. By confining broadband light at the TMD monolayer, our one type of Mie resonator on different TMDs enables enhanced light-matter interactions with multiple excitonic states simultaneously. We demonstrate multi-Rabi splittings and robust polaritonic photoluminescence in monolayer WSe2, WS2, and MoS2. The hybrid system also shows the potential to approach the ultrastrong coupling regime.

Dispersion engineered metasurfaces for broadband, high-NA, high-efficiency, dual-polarization analog image processing

Optical metasurfaces performing analog image processing - such as spatial differentiation and edge detection - hold the potential to reduce processing times and power consumption, while avoiding bulky 4 F lens systems. However, current designs have been suffering from trade-offs between spatial resolution, throughput, polarization asymmetry, operational bandwidth, and isotropy. Here, we show that dispersion engineering provides an elegant way to design metasurfaces where all these critical metrics are simultaneously optimized. We experimentally demonstrate silicon metasurfaces performing isotropic and dual-polarization edge detection, with numerical apertures above 0.35 and spectral bandwidths of 35 nm around 1500 nm. Moreover, we introduce quantitative metrics to assess the efficiency of these devices. Thanks to the low loss nature and dual-polarization response, our metasurfaces feature large throughput efficiencies, approaching the theoretical maximum for a given NA. Our results pave the way for low-loss, high-efficiency and broadband optical computing and image processing with free-space metasurfaces.

Sound non-reciprocity based on synthetic magnetism

Synthetic magnetism has been recently realized using spatiotemporal modulation patterns, producing non-reciprocal steering of charge-neutral particles such as photons and phonons. Here, we design and experimentally demonstrate a non-reciprocal acoustic system composed of three compact cavities interlinked with both dynamic and static couplings, in which phase-correlated modulations induce a synthetic magnetic flux that breaks time-reversal symmetry. Within the rotating wave approximation, the transport properties of the system are controlled to efficiently realize large non-reciprocal acoustic transport. By optimizing the coupling strengths and modulation phases, we achieve frequency-preserved unidirectional transport with 45-dB isolation ratio and 0.85 forward transmission. Our results open to the realization of acoustic non-reciprocal technologies with high efficiency and large isolation, and offer a route towards Floquet topological insulators for sound.

Controlling the propagation asymmetry of hyperbolic shear polaritons in beta-gallium oxide

Structural anisotropy in crystals is crucial for controlling light propagation, particularly in the infrared spectral regime where optical frequencies overlap with crystalline lattice resonances, enabling light-matter coupled quasiparticles called phonon polaritons (PhPs). Exploring PhPs in anisotropic materials like hBN and MoO3 has led to advancements in light confinement and manipulation. In a recent study, PhPs in the monoclinic crystal β-Ga2O3 (bGO) were shown to exhibit strongly asymmetric propagation with a frequency dispersive optical axis. Here, using scanning near-field optical microscopy (s-SNOM), we directly image the symmetry-broken propagation of hyperbolic shear polaritons in bGO. Further, we demonstrate the control and enhancement of shear-induced propagation asymmetry by varying the incident laser orientation and polariton momentum using different sizes of nano-antennas. Finally, we observe significant rotation of the hyperbola axis by changing the frequency of incident light. Our findings lay the groundwork for the widespread utilization and implementation of polaritons in low-symmetry crystals.

Adiabatic topological photonic interfaces

Topological phases of matter have been attracting significant attention across diverse fields, from inherently quantum systems to classical photonic and acoustic metamaterials. In photonics, topological phases offer resilience and bring novel opportunities to control light with pseudo-spins. However, topological photonic systems can suffer from limitations, such as breakdown of topological properties due to their symmetry-protected origin and radiative leakage. Here we introduce adiabatic topological photonic interfaces, which help to overcome these issues. We predict and experimentally confirm that topological metasurfaces with slowly varying synthetic gauge fields significantly improve the guiding features of spin-Hall and valley-Hall topological structures commonly used in the design of topological photonic devices. Adiabatic variation in the domain wall profiles leads to the delocalization of topological boundary modes, making them less sensitive to details of the lattice, perceiving the structure as an effectively homogeneous Dirac metasurface. As a result, the modes showcase improved bandgap crossing, longer radiative lifetimes and propagation distances.

Overcoming Intensity Saturation in Nonlinear Multiple-Quantum-Well Metasurfaces for High-Efficiency Frequency Upconversion

Engineered intersubband transitions in semiconductor heterostructures featuring multiple quantum wells (MQWs) are shown to support record-high second-order nonlinear susceptibilities. By integrating these materials in metasurfaces with tailored optical resonances, it is possible to further enhance photonic interactions, yielding giant nonlinear responses in ultrathin devices. These metasurfaces form a promising platform for efficient nonlinear processes, including frequency upconversion of low-intensity thermal infrared radiation and harmonic generation, free of phase-matching constraints intrinsic to bulk nonlinear crystals. However, nonlinear saturation at moderately large pump intensities due to the transfer of electron population into excited subbands facilitated by strongly enhanced light-matter interactions in metasurfaces fundamentally limits their overall efficiency for various nonlinear processes. Here, the saturation limits of nonlinear MQW-based metasurfaces for mid-infrared frequency upconversion are significantly extended by optimizing their designs for excitation with a strong pump coherently coupled with unpopulated upper electron subbands. This counterintuitive pumping scheme, combined with tailored material and photonic engineering of the metasurface, avoids saturation at practical levels of continuous-wave pump intensities, yielding significantly larger upconversion efficiencies than in conventional approaches. The present results open new opportunities for nonlinear metasurfaces, less limited by saturation mechanisms, with important implications for night-vision imaging and compact nonlinear wave mixing systems.

Magneto-optics in a van der Waals magnet tuned by self-hybridized polaritons

Controlling quantum materials with light is of fundamental and technological importance. By utilizing the strong coupling of light and matter in optical cavities1-3, recent studies were able to modify some of their most defining features4-6. Here we study the magneto-optical properties of a van der Waals magnet that supports strong coupling of photons and excitons even in the absence of external cavity mirrors. In this material-the layered magnetic semiconductor CrSBr-emergent light-matter hybrids called polaritons are shown to substantially increase the spectral bandwidth of correlations between the magnetic, electronic and optical properties, enabling largely tunable optical responses to applied magnetic fields and magnons. Our results highlight the importance of exciton-photon self-hybridization in van der Waals magnets and motivate novel directions for the manipulation of quantum material properties by strong light-matter coupling.

Metasurfaces for next-generation wireless communication systems

Tailored time variations, nonlinearities and active elements can endow metasurfaces with unique opportunities for next-generation wireless communication systems, enriching the growing platform of reconfigurable intelligent surfaces.

Arbitrary aperture synthesis with nonlocal leaky-wave metasurface antennas

The emergence of new technological needs in 5 G/6 G networking and broadband satellite internet access amplifies the demand for innovative wireless communication hardware, including high-performance low-profile transceivers. In this context, antennas based on metasurfaces - artificial surfaces engineered to manipulate electromagnetic waves at will - represent highly promising solutions. In this article, we introduce leaky-wave metasurface antennas operating at micro/millimeter-wave frequencies that are designed using the principles of quasi-bound states in the continuum, exploiting judiciously tailored spatial symmetries that enable fully customized radiation. Specifically, we unveil additional degrees of control over leaky-wave radiation by demonstrating pointwise control of the amplitude, phase and polarization state of the metasurface aperture fields by carefully breaking relevant symmetries with tailored perturbations. We design and experimentally demonstrate metasurface antenna prototypes showcasing a variety of functionalities advancing capabilities in wireless communications, including single-input multi-output and multi-input multi-output near-field focusing, as well as far-field beam shaping.

Self-bridging metamaterials surpassing the theoretical limit of Poisson's ratios

A hallmark of mechanical metamaterials has been the realization of negative Poisson's ratios, associated with auxeticity. However, natural and engineered Poisson's ratios obey fundamental bounds determined by stability, linearity and thermodynamics. Overcoming these limits may substantially extend the range of Poisson's ratios realizable in mechanical systems, of great interest for medical stents and soft robots. Here, we demonstrate freeform self-bridging metamaterials that synthesize multi-mode microscale levers, realizing Poisson's ratios surpassing the values allowed by thermodynamics in linear materials. Bridging slits between microstructures via self-contacts yields multiple rotation behaviors of microscale levers, which break the symmetry and invariance of the constitutive tensors under different load scenarios, enabling inaccessible deformation patterns. Based on these features, we unveil a bulk mode that breaks static reciprocity, providing an explicit and programmable way to manipulate the non-reciprocal transmission of displacement fields in static mechanics. Besides non-reciprocal Poisson's ratios, we also realize ultra-large and step-like values, which make metamaterials exhibit orthogonally bidirectional displacement amplification, and expansion under both tension and compression, respectively.

Exceptional points and non-Hermitian photonics at the nanoscale

Exceptional points (EPs) arising in non-Hermitian systems have led to a variety of intriguing wave phenomena, and have been attracting increased interest in various physical platforms. In this Review, we highlight the latest fundamental advances in the context of EPs in various nanoscale systems, and overview the theoretical progress related to EPs, including higher-order EPs, bulk Fermi arcs and Weyl exceptional rings. We peek into EP-associated emerging technologies, in particular focusing on the influence of noise for sensing near EPs, improving the efficiency in asymmetric transmission based on EPs, optical isolators in nonlinear EP systems and novel concepts to implement EPs in topological photonics. We also discuss the constraints and limitations of the applications relying on EPs, and offer parting thoughts about promising ways to tackle them for advanced nanophotonic applications.

Interaction-driven transport of dark excitons in 2D semiconductors with phonon-mediated optical readout

The growing field of quantum information technology requires propagation of information over long distances with efficient readout mechanisms. Excitonic quantum fluids have emerged as a powerful platform for this task due to their straightforward electro-optical conversion. In two-dimensional transition metal dichalcogenides, the coupling between spin and valley provides exciting opportunities for harnessing, manipulating, and storing bits of information. However, the large inhomogeneity of single layers cannot be overcome by the properties of bright excitons, hindering spin-valley transport. Nonetheless, the rich band structure supports dark excitonic states with strong binding energy and longer lifetime, ideally suited for long-range transport. Here we show that dark excitons can diffuse over several micrometers and prove that this repulsion-driven propagation is robust across non-uniform samples. The long-range propagation of dark states with an optical readout mediated by chiral phonons provides a new concept of excitonic devices for applications in both classical and quantum information technology.

Multiresonant Nonlocal Metasurfaces

Optical metasurfaces supporting localized resonances have become a versatile platform for shaping the wavefront of light, but their low quality (Q-) factor modes inevitably modify the wavefront over extended momentum and frequency space, resulting in limited spectral and angular control. In contrast, periodic nonlocal metasurfaces have been providing great flexibility for both spectral and angular selectivity but with limited spatial control. Here, we introduce multiresonant nonlocal metasurfaces capable of shaping the spatial properties of light using several resonances with widely disparate Q-factors. In contrast to previous designs, the narrowband resonant transmission punctuates a broadband resonant reflection window enabled by a highly symmetric array, achieving simultaneous spectral filtering and wavefront shaping in the transmission mode. Through rationally designed perturbations, we realize nonlocal flat lenses suitable as compact band-pass imaging devices, ideally suited for microscopy. We further employ modified topology optimization to demonstrate high-quality-factor metagratings for extreme wavefront transformations with large efficiency.

Ultrahigh-Q guided mode resonances in an All-dielectric metasurface

High quality(Q) factor optical resonators are indispensable for many photonic devices. While very large Q-factors can be obtained theoretically in guided-mode settings, free-space implementations suffer from various limitations on the narrowest linewidth in real experiments. Here, we propose a simple strategy to enable ultrahigh-Q guided-mode resonances by introducing a patterned perturbation layer on top of a multilayer-waveguide system. We demonstrate that the associated Q-factors are inversely proportional to the perturbation squared while the resonant wavelength can be tuned through material or structural parameters. We experimentally demonstrate such high-Q resonances at telecom wavelengths by patterning a low-index layer on top of a 220 nm silicon on insulator substrate. The measurements show Q-factors up to 2.39 × 10⁵, comparable to the largest Q-factor obtained by topological engineering, while the resonant wavelength is tuned by varying the lattice constant of the top perturbation layer. Our results hold great promise for exciting applications like sensors and filters.

Generalized surface admittance equivalence principle for super-scattering in dielectric particles

A general formulation for controlling the external scattering coefficients of cylindrical harmonics is presented, generalizing previous results for cloaking of a bare dielectric particle. By inserting a suitable surface admittance at the boundary between a dielectric body and the background region, cylindrical harmonic waves can be enhanced by tailoring the admittance value. Two separate limiting cases for super-scattering features are presented and compared against the same bare particle reference case, providing insights on how to enhance the multi-harmonic scattering pattern. Using this formulation, super-scattering systems can be created, which are suitable for future implementation using active or passive thin metasurfaces.

Topological Metamaterials

The topological properties of an object, associated with an integer called the topological invariant, are global features that cannot change continuously but only through abrupt variations, hence granting them intrinsic robustness. Engineered metamaterials (MMs) can be tailored to support highly nontrivial topological properties of their band structure, relative to their electronic, electromagnetic, acoustic and mechanical response, representing one of the major breakthroughs in physics over the past decade. Here, we review the foundations and the latest advances of topological photonic and phononic MMs, whose nontrivial wave interactions have become of great interest to a broad range of science disciplines, such as classical and quantum chemistry. We first introduce the basic concepts, including the notion of topological charge and geometric phase. We then discuss the topology of natural electronic materials, before reviewing their photonic/phononic topological MM analogues, including 2D topological MMs with and without time-reversal symmetry, Floquet topological insulators, 3D, higher-order, non-Hermitian and nonlinear topological MMs. We also discuss the topological aspects of scattering anomalies, chemical reactions and polaritons. This work aims at connecting the recent advances of topological concepts throughout a broad range of scientific areas and it highlights opportunities offered by topological MMs for the chemistry community and beyond.

Observation of directional leaky polaritons at anisotropic crystal interfaces

Extreme anisotropy in some polaritonic materials enables light propagation with a hyperbolic dispersion, leading to enhanced light-matter interactions and directional transport. However, these features are typically associated with large momenta that make them sensitive to loss and poorly accessible from far-field, being bound to the material interface or volume-confined in thin films. Here, we demonstrate a new form of directional polaritons, leaky in nature and featuring lenticular dispersion contours that are neither elliptical nor hyperbolic. We show that these interface modes are strongly hybridized with propagating bulk states, sustaining directional, long-range, sub-diffractive propagation at the interface. We observe these features using polariton spectroscopy, far-field probing and near-field imaging, revealing their peculiar dispersion, and - despite their leaky nature - long modal lifetime. Our leaky polaritons (LPs) nontrivially merge sub-diffractive polaritonics with diffractive photonics onto a unified platform, unveiling opportunities that stem from the interplay of extreme anisotropic responses and radiation leakage.

Leaky-wave metasurfaces for integrated photonics

Metasurfaces have been rapidly advancing our command over the many degrees of freedom of light; however, so far, they have been mostly limited to manipulating light in free space. Metasurfaces integrated on top of guided-wave photonic systems have been explored to control the scattering of light off-chip with enhanced functionalities-namely, the point-by-point manipulation of amplitude, phase or polarization. However, these efforts have so far been limited to controlling one or two optical degrees of freedom at best, as well as device configurations much more complex compared with conventional grating couplers. Here we introduce leaky-wave metasurfaces, which are based on symmetry-broken photonic crystal slabs that support quasi-bound states in the continuum. This platform has a compact form factor equivalent to the one of grating couplers, but it provides full command over the amplitude, phase and polarization (four optical degrees of freedom) across large apertures. We present devices for phase and amplitude control at a fixed polarization state, and devices controlling all the four optical degrees of freedom for operation at a wavelength of 1.55 μm. Merging the fields of guided and free-space optics through the hybrid nature of quasi-bound states in the continuum, our leaky-wave metasurfaces may find applications in imaging, communications, augmented reality, quantum optics, LIDAR and integrated photonic systems.

Remote Water-to-Air Eavesdropping with a Phase-Engineered Impedance Matching Metasurface

Efficiently receiving underwater sound remotely from air is a long-standing challenge in acoustics hindered by the large impedance mismatch at the water-air interface. Here, a phase-engineered water-air impedance matching metasurface is proposed and experimentally demonstrated for remote and efficient water-to-air eavesdropping. The judiciously designed metasurface with near-unity transmission efficiency, long monitoring distance, and high mechanical stiffness is capable of making the water-air interface acoustically transparent and, at the same time, freewheelingly patterning the transmitted wavefront. This enables efficient control over the effective spatial location of a distant airborne sensor such that it can measure underwater signals with large signal-to-noise ratios as if placed close to the physical underwater source. Such airborne eavesdropping of underwater sound is experimentally demonstrated with a measured sensitivity enhancement of nearly 10⁴ at 8 kHz, far from achievable with the current state-of-the-art methods. Moreover, the opportunities of using the proposed metasurface for cross-media orbital-angular-momentum-multiplexed communication and underwater acoustic window are also demonstrated. This metasurface opens new avenues for communication and sensing in inhomogeneities with totally reflective interfaces, which may be translated to nano-optics and radio frequencies.

Solving integral equations in free space with inverse-designed ultrathin optical metagratings

As standard microelectronic technology approaches fundamental limitations in speed and power consumption, novel computing strategies are strongly needed. Analogue optical computing enables the processing of large amounts of data at a negligible energy cost and high speeds. Based on these principles, ultrathin optical metasurfaces have been recently explored to process large images in real time, in particular for edge detection. By incorporating feedback, it has also recently been shown that metamaterials can be tailored to solve complex mathematical problems in the analogue domain, although these efforts have so far been limited to guided-wave systems and bulky set-ups. Here, we present an ultrathin Si metasurface-based platform for analogue computing that is able to solve Fredholm integral equations of the second kind using free-space visible radiation. A Si-based metagrating was inverse-designed to implement the scattering matrix synthesizing a prescribed kernel corresponding to the mathematical problem of interest. Next, a semitransparent mirror was incorporated into the sample to provide adequate feedback and thus perform the required Neumann series, solving the corresponding equation in the analogue domain at the speed of light. Visible wavelength operation enables a highly compact, ultrathin device that can be interrogated from free space, implying high processing speeds and the possibility of on-chip integration.

The Bulk van der Waals Layered Magnet CrSBr is a Quasi-1D Material

Correlated quantum phenomena in one-dimensional (1D) systems that exhibit competing electronic and magnetic order are of strong interest for the study of fundamental interactions and excitations, such as Tomonaga-Luttinger liquids and topological orders and defects with properties completely different from the quasiparticles expected in their higher-dimensional counterparts. However, clean 1D electronic systems are difficult to realize experimentally, particularly for magnetically ordered systems. Here, we show that the van der Waals layered magnetic semiconductor CrSBr behaves like a quasi-1D material embedded in a magnetically ordered environment. The strong 1D electronic character originates from the Cr-S chains and the combination of weak interlayer hybridization and anisotropy in effective mass and dielectric screening, with an effective electron mass ratio of m_X^e/m_Y^e ∼ 50. This extreme anisotropy experimentally manifests in strong electron-phonon and exciton-phonon interactions, a Peierls-like structural instability, and a Fano resonance from a van Hove singularity of similar strength to that of metallic carbon nanotubes. Moreover, because of the reduced dimensionality and interlayer coupling, CrSBr hosts spectrally narrow (1 meV) excitons of high binding energy and oscillator strength that inherit the 1D character. Overall, CrSBr is best understood as a stack of weakly hybridized monolayers and appears to be an experimentally attractive candidate for the study of exotic exciton and 1D-correlated many-body physics in the presence of magnetic order.

Photonic Dirac cavities with spatially varying mass term

In recent years, photonics has proven itself as an excellent platform for emulation of relativistic phenomena. Here, we show an example of relativistic-like trapping in photonic system that realizes Dirac-like dispersion with spatially inhomogeneous mass term. The modes trapped by such cavities, their energy levels, and corresponding orbitals are then characterized through optical imaging in real and momentum space. The fabricated cavities host a hierarchy of photonic modes with distinct radiation profiles directly analogous to various atomic orbitals endowed with unique characteristics, such as pseudo-particle-hall symmetry and spin degeneracy, and they carry topological charge which gives rise to radiative profiles with angular momentum. We demonstrate that these modes can be directionally excited by pseudo-spin-polarized boundary states. In addition to the fundamental interest in the structure of these pseudo-relativistic orbitals, the proposed system offers a route for designing new types of nanophotonic devices, spin-full resonators and topological light sources compatible with integrated photonics platforms.

Coherent Control of Topological States in an Integrated Waveguide Lattice

Topological photonics holds the promise for enhanced robustness of light localization and propagation enabled by the global symmetries of the system. While traditional designs of topological structures rely on lattice symmetries, there is an alternative strategy based on accidentally degenerate modes of the individual meta-atoms. Using this concept, we experimentally realize topological edge state in an array of silicon nanostructured waveguides, each hosting a pair of degenerate modes at telecom wavelengths. Exploiting the hybrid nature of the topological mode, we implement its coherent control by adjusting the phase between the degenerate modes and demonstrating selective excitation of bulk or edge states. The resulting field distribution is imaged via third harmonic generation showing the localization of topological modes as a function of the relative phase of the excitations. Our results highlight the impact of engineered accidental degeneracies on the formation of topological phases, extending the opportunities stemming from topological nanophotonic systems.

Stationary Charge Radiation in Anisotropic Photonic Time Crystals

Time metamaterials offer a great potential for wave manipulation, drawing increasing attention in recent years. Here, we explore the exotic wave dynamics of an anisotropic photonic time crystal (APTC) formed by an anisotropic medium whose optical properties are uniformly and periodically changed in time. Based on a temporal transfer matrix formalism, we show that a stationary charge embedded in an APTC emits radiation, in contrast to the case of isotropic photonic time crystals, and its distribution in momentum space is controlled by the APTC band structure. Our approach extends the functionalities of time metamaterials, offering new opportunities for radiation generation and control, with implications for both classical and quantum applications.

Sensing the Local Magnetic Environment through Optically Active Defects in a Layered Magnetic Semiconductor

Atomic-level defects in van der Waals (vdW) materials are essential building blocks for quantum technologies and quantum sensing applications. The layered magnetic semiconductor CrSBr is an outstanding candidate for exploring optically active defects because of a direct gap, in addition to a rich magnetic phase diagram, including a recently hypothesized defect-induced magnetic order at low temperature. Here, we show optically active defects in CrSBr that are probes of the local magnetic environment. We observe a spectrally narrow (1 meV) defect emission in CrSBr that is correlated with both the bulk magnetic order and an additional low-temperature, defect-induced magnetic order. We elucidate the origin of this magnetic order in the context of local and nonlocal exchange coupling effects. Our work establishes vdW magnets like CrSBr as an exceptional platform to optically study defects that are correlated with the magnetic lattice. We anticipate that controlled defect creation allows for tailor-made complex magnetic textures and phases with direct optical access.

Real-space nanoimaging of hyperbolic shear polaritons in a monoclinic crystal

Various optical crystals possess permittivity components of opposite signs along different principal directions in the mid-infrared regime, exhibiting exotic anisotropic phonon resonances. Such materials with hyperbolic polaritons-hybrid light-matter quasiparticles with open isofrequency contours-feature large-momenta optical modes and wave confinement that make them promising for nanophotonic on-chip technologies. So far, hyperbolic polaritons have been observed and characterized in crystals with high symmetry including hexagonal (boron nitride), trigonal (calcite) and orthorhombic (α-MoO₃ or α-V₂O₅) crystals, where they obey certain propagation patterns. However, lower-symmetry materials such as monoclinic crystals were recently demonstrated to offer richer opportunities for polaritonic phenomena. Here, using scanning near-field optical microscopy, we report the direct real-space nanoscale imaging of symmetry-broken hyperbolic phonon polaritons in monoclinic CdWO₄ crystals, and showcase inherently asymmetric polariton excitation and propagation associated with the nanoscale shear phenomena. We also introduce a quantitative theoretical model to describe these polaritons that leads to schemes to enhance crystal asymmetry via the damping loss of phonon modes. Ultimately, our findings show that polaritonic nanophotonics is attainable using natural materials with low symmetry, favouring a versatile and general way to manipulate light at the nanoscale.

Temporal optical activity and chiral time-interfaces [Invited]

Time-interfaces, at which the optical properties of a medium undergo abrupt and spatially uniform changes, have attracted surging interest in optics and wave physics. In this work, we study wave scattering at time-interfaces involving chiral media. Dual to spatial interfaces involving chiral media, we show that a propagating wave is split upon a chiral time-interface into two orthogonal circular polarization waves oscillating at different frequencies. We formulate the temporal scattering boundary-value problem at such time-interfaces, and then demonstrate the effect of temporal optical activity through a chiral time-slab. The effect of material dispersion is also analyzed, highlighting interesting opportunities in which multiple scattered waves emerge form the time-interface and interfere. Our results pave the way towards time-metamaterials encompassing chirality as an additional degree of freedom for wave manipulation, offering opportunities for temporal circular dichroism and negative refraction at time-interfaces.

Observation of localized magnetic plasmon skyrmions

Optical skyrmions have recently been constructed by tailoring vectorial near-field distributions through the interference of multiple surface plasmon polaritons, offering promising features for advanced information processing, transport and storage. Here, we provide experimental demonstration of electromagnetic skyrmions based on magnetic localized spoof plasmons (LSP) showing large topological robustness against continuous deformations, without stringent external interference conditions. By directly measuring the spatial profile of all three vectorial magnetic fields, we reveal multiple π-twist target skyrmion configurations mapped to multi-resonant near-equidistant LSP eigenmodes. The real-space skyrmion topology is robust against deformations of the meta-structure, demonstrating flexible skyrmionic textures for arbitrary shapes. The observed magnetic LSP skyrmions pave the way to ultra-compact and robust plasmonic devices, such as flexible sensors, wearable electronics and ultra-compact antennas.

Beyond Bounds on Light Scattering with Complex Frequency Excitations

Light scattering is one of the most established wave phenomena in optics, lying at the heart of light-matter interactions and of crucial importance for nanophotonic applications. Passivity, causality, and energy conservation imply strict bounds on the degree of control over scattering from small particles, with implications on the performance of many optical devices. Here, we demonstrate that these bounds can be surpassed by considering excitations at complex frequencies, yielding extreme scattering responses as tailored nanoparticles reach a quasi-steady-state regime. These mechanisms can be used to engineer light scattering of nanostructures beyond conventional limits for noninvasive sensing, imaging, and nanoscale light manipulation.

Synthetic Pseudo-Spin-Hall effect in acoustic metamaterials

While vector fields naturally offer additional degrees of freedom for emulating spin, acoustic pressure field is scalar in nature, and it requires engineering of synthetic degrees of freedom by material design. Here we experimentally demonstrate the control of sound waves by using two types of engineered acoustic systems, where synthetic pseudo-spin emerges either as a consequence of the evanescent nature of the field or due to lattice symmetry. First, we show that evanescent sound waves in perforated films possess transverse angular momentum locked to their propagation direction which enables their directional excitation. Second, we demonstrate that lattice symmetries of an acoustic kagome lattice also enable a synthetic transverse pseudo-spin locked to the linear momentum, enabling control of the propagation of modes both in the bulk and along the edges. Our results open a new degree of control of radiation and propagation of acoustic waves thus offering new design approaches for acoustic devices.

Controlling emission and propagation of acoustic waves offers new design opportunities for acoustic devices. Here the authors demonstrate such controls thanks to the emergence of a synthetic pseudo-spin in two-dimensional acoustic metamaterial.

Riemann-Encircling Exceptional Points for Efficient Asymmetric Polarization-Locked Devices

Dynamically encircling exceptional points (EPs) have unveiled intriguing chiral dynamics in photonics. However, the traditional approach based on an open manifold of Hamiltonian parameter space fails to explore trajectories that pass through an infinite boundary. Here, by mapping the full parameter space onto a closed manifold of the Riemann sphere, we introduce a framework to describe encircling-EP loops. We demonstrate that an encircling trajectory crossing the north vertex can realize near-unity asymmetric transmission. An efficient gain-free, broadband asymmetric polarization-locked device is realized by mapping the encircling path onto L-shaped silicon waveguides.

Topological sound in two dimensions

Topology is the branch of mathematics studying the properties of an object that are preserved under continuous deformations. Quite remarkably, the powerful theoretical tools of topology have been applied over the past few years to study the electronic band structure of crystals. Topological band theory can explain and predict topological phase transitions in a material, and the unusual robustness of certain band structure shapes, such as Dirac cones, against small perturbations. These findings have also unveiled a new phase of matter-topological insulators-whose exotic transport properties at their boundaries are topologically protected against imperfections and disorder. The fascinating features of topological boundary states have triggered the search for their analogs in classical wave physics. Here, we focus on the peculiar features of two-dimensional topological insulators for sound and mechanical waves. Two-dimensional Dirac cones and phononic topological insulators can emerge under certain conditions in periodic acoustic metamaterials, demonstrating great potential for acoustic and mechanical systems to demonstrate, over a tabletop platform, complex fundamental phenomena driven by topological concepts. In addition, these discoveries offer a direct path toward new technologies for enhanced sound control and manipulation.

Multifunctional resonant wavefront-shaping meta-optics based on multilayer and multi-perturbation nonlocal metasurfaces

Photonic devices rarely provide both elaborate spatial control and sharp spectral control over an incoming wavefront. In optical metasurfaces, for example, the localized modes of individual meta-units govern the wavefront shape over a broad bandwidth, while nonlocal lattice modes extended over many unit cells support high quality-factor resonances. Here, we experimentally demonstrate nonlocal dielectric metasurfaces in the near-infrared that offer both spatial and spectral control of light, realizing metalenses focusing light exclusively over a narrowband resonance while leaving off-resonant frequencies unaffected. Our devices attain this functionality by supporting a quasi-bound state in the continuum encoded with a spatially varying geometric phase. We leverage this capability to experimentally realize a versatile platform for multispectral wavefront shaping where a stack of metasurfaces, each supporting multiple independently controlled quasi-bound states in the continuum, molds the optical wavefront distinctively at multiple wavelengths and yet stay transparent over the rest of the spectrum. Such a platform is scalable to the visible for applications in augmented reality and transparent displays.

Reconfigurable hyperbolic polaritonics with correlated oxide metasurfaces

Polaritons enable subwavelength confinement and highly anisotropic flows of light over a wide spectral range, holding the promise for applications in modern nanophotonic and optoelectronic devices. However, to fully realize their practical application potential, facile methods enabling nanoscale active control of polaritons are needed. Here, we introduce a hybrid polaritonic-oxide heterostructure platform consisting of van der Waals crystals, such as hexagonal boron nitride (hBN) or alpha-phase molybdenum trioxide (α-MoO₃), transferred on nanoscale oxygen vacancy patterns on the surface of prototypical correlated perovskite oxide, samarium nickel oxide, SmNiO₃ (SNO). Using a combination of scanning probe microscopy and infrared nanoimaging techniques, we demonstrate nanoscale reconfigurability of complex hyperbolic phonon polaritons patterned at the nanoscale with high resolution. Hydrogenation and temperature modulation allow spatially localized conductivity modulation of SNO nanoscale patterns, enabling robust real-time modulation and nanoscale reconfiguration of hyperbolic polaritons. Our work paves the way towards nanoscale programmable metasurface engineering for reconfigurable nanophotonic applications.

Phonon polaritons in anisotropic van der Waals materials enable subwavelength confinement and controllable flow of light at the nanoscale. Here, the authors exploit correlated perovskite oxide (SmNiO₃) substrates with tunable conductivity to obtain real-time modulation and nanoscale reconfiguration of hyperbolic polaritons in hBN and α-MoO₃ crystals.

Unidirectionally excited phonon polaritons in high-symmetry orthorhombic crystals

Advanced control over the excitation of ultraconfined polaritons-hybrid light and matter waves-empowers unique opportunities for many nanophotonic functionalities, e.g., on-chip circuits, quantum information processing, and controlling thermal radiation. Recent work has shown that highly asymmetric polaritons are directly governed by asymmetries in crystal structures. Here, we experimentally demonstrate extremely asymmetric and unidirectional phonon polariton (PhP) excitation via directly patterning high-symmetry orthorhombic van der Waals (vdW) crystal α-MoO₃. This phenomenon results from symmetry breaking of momentum matching in polaritonic diffraction in vdW materials. We show that the propagation of PhPs can be versatile and robustly tailored via structural engineering, while PhPs in low-symmetry (e.g., monoclinic and triclinic) crystals are largely restricted by their naturally occurring permittivities. Our work synergizes grating diffraction phenomena with the extreme anisotropy of high-symmetry vdW materials, enabling unexpected control of infrared polaritons along different pathways and opening opportunities for applications ranging from on-chip photonics to directional heat dissipation.

Planar chiral metasurfaces with maximal and tunable chiroptical response driven by bound states in the continuum

Optical metasurfaces with high quality factors (Q-factors) of chiral resonances can boost substantially light-matter interaction for various applications of chiral response in ultrathin, active, and nonlinear metadevices. However, current approaches lack the flexibility to enhance and tune the chirality and Q-factor simultaneously. Here, we suggest a design of chiral metasurface supporting bound state in the continuum (BIC) and demonstrate experimentally chiroptical responses with ultra-high Q-factors and near-perfect circular dichroism (CD = 0.93) at optical frequencies. We employ the symmetry-reduced meta-atoms with high birefringence supporting winding elliptical eigenstate polarizations with opposite helicity. It provides a convenient way for achieving the maximal planar chirality tuned by either breaking in-plane structure symmetry or changing illumination angle. Beyond linear CD, we also achieved strong near-field enhancement CD and near-unitary nonlinear CD in the same planar chiral metasurface design with circular eigen-polarization. Sharply resonant chirality realized in planar metasurfaces promises various practical applications including chiral lasers and chiral nonlinear filters.

Here, the authors employ the physics of chiral bound states in the continuum and suggest planar chiral metasurfaces with simultaneous ultrahigh quality factor and near-perfect circular dichroism in both linear regime and nonlinear regime.